Blood Supply to the Brain

The anterior 2/3 of the brain is supplied by branches of the internal carotid artery, whose terminal branches form the anterior and middle cerebral arteries.

The vertebral arteries branch off the subclavian arteries and through small openings in the transverse processes of the cervical vertebrae. They merge to form the basilar artery supplying the cerebellum, brain stem and the posterior cerebrum via the posterior cerebral arteries.

The Circle of Willis has small connecting vessels between the three major cerebral vessels. Blood can change direction within the circle for collateral blood flow if needed. There can be significant variation in the form of the Circle of Willis from one individual to another.

Tissues supplied by the tiny terminal branches of vessels are known as watershed regions, and are vulnerable to damage during periods of low perfusion or oxygenation.